1. Field of the Invention
This invention relates to adaptive equalization and more particularly relates to adapting equalization coefficients using leakage.
2. Description of the Related Art
Data processing systems often use magnetic tape for high volume, low cost data storage. For example, a data processing system may backup the data from a data storage subsystem comprising a plurality of hard disk drives to magnetic tape. Large volumes of infrequently used data may also be stored to magnetic tape. For example, data intensive geological study data, meteorological data, or the like may be cost effectively archived on magnetic tape.
A user or software application may retrieve data from the magnetic tape by mounting the magnetic tape on a magnetic tape drive and reading the data from the magnetic tape. The magnetic tape drive reads the magnetic tape by sensing magnetic polarization changes on the magnetic tape that encode the data and generates an analog signal from the magnetic polarization changes that embodies the data. The analog read signal is sampled and car converted to a plurality of digital values that form a digital read signal.
The digital read signal comprises a plurality of frequency components, each with a magnitude and phase characteristic. Variations in the magnitude and phase characteristics of the frequency components increase the difficulty of recognizing the data in the digital read signal.
As a result, the magnetic tape drive typically equalizes or adjusts the magnitude and phase characteristic of each frequency component so that the data may be more easily recognized and recovered. The magnetic tape drive often equalizes the digital read signal by storing a plurality of digital values in a delay line. The digital values are sampled for a plurality of instances of the read signal with an analog-to-digital converter operating at a sampling frequency that is not synchronized with respect to the duration of the bits stored on the tape medium. Each stored digital value or tap signal is multiplied by a coefficient and the sum of the tap coefficient products forms an equalized signal value for a specified instance of the asynchronous sampling clock.
The data may have originally been written by one or more of a variety of magnetic tape drives from a variety of manufacturers. In addition, each magnetic tape may have originally been written under a wide range of environmental conditions. As a result, when magnetic tapes are read, magnetic tape read signals often exhibit a wide range of characteristics. As a result, the magnetic tape drive must often dynamically adjust the tap coefficients used to equalize the read back signal to compensate for differences in the read signal.
Unfortunately, adapting the coefficients of the asynchronous equalizer may cause the equalization function to become unstable. For example, adapting the coefficients may drive one or more coefficients to an excessive value that destabilizes the equalization function. Therefore, some coefficient values may be frozen at specified values. Freezing coefficients reduces the probability that the equalization function will become unstable, but also reduces the equalization function's ability to adapt to differing read signal characteristics.
In addition, adapting equalization coefficients of an asynchronous equalizer often increases the strength of higher frequencies that do not include significant signal elements. As a result, the high-frequency noise of the read signal is increased, reducing the tape drives ability to recognize and retrieve data from the read signal.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that adapt equalization coefficients while maintaining equalization function stability for an asynchronous equalizer. Beneficially, such an apparatus, system, and method would increase the asynchronous equalization function's ability to adapt to different read signals.